Sensing circuits for detecting wave signals, and more specifically sine wave signals, are well known in the art. They are often used as activity detector in telephony central units to initiate the set up of a telephony system being in sleep mode upon receipt of a request from a user, where this request is represented by a sine wave signal. The latter sleep mode is a low power mode limitations the power consumption at the user side, but especially at the central unit side where a relatively large amount of lines have to be powered. This mode is especially suited for systems wherein broadband services are offered on twisted pair cables since these services consume a lot of power due to the high operation speeds.
However, the existing sensing circuits are not suited for mixed systems for low and high frequency signals, such as ADSL systems as described in the ANSI draft standard "Asymetric Digital Subscriber Line (ADSL) Metallic Interface Specification", due to the presence of offsets and low frequency signals e.g. Plain Old Telephony Service or POTS signals.